The present invention relates to a hot cathode in a wire form useful in such devices as vacuum tubes, CRTs and fluorescent display tubes.
Various devices employing hot cathodes in a wire form have been proposed and a display device of the panel type is shown schematically in cross section in FIG. 2 (Unexamined Published Japanese Patent Application No. 84744/1985). As shown, this device comprises an insulating substrate 1, a plurality of metallic supports 2 provided on the substrate 1 at given intervals, a cathode wire 3 that holds an electron-emitting material at given intervals to form cathodes 4b and which spans said supports 2, control electrodes 5 that are provided on the substrate 1 at positions corresponding to said cathodes 4b, a grid electrode 6 that is provided above the cathode wire 3 and which is provided with through-holes 6a at positions corresponding to the cathodes 4b, and an anode 8 that is placed above said grid electrode 6 and which is coated with a phosphor 7 at positions corresponding to said cathodes 4b, said grid electrode 6 and the anode 8 being separated by a given distance in the vertical direction.
The cathode wire 3 is made of tungsten and the cathodes 4b are formed of a ternary carbonate of barium, strontium and calcium [(Ba,Sr,Ca)CO.sub.3 ] that is deposited on the surface of the wire 3 by a suitable method such as electrodeposition or coating and which is thermally decomposed to an oxide form [(Ba,Sr,Ca)0] during evacuation of the chamber of the display device. During the thermal decomposition, BaO in the electron-emitting material is reduced to generate excess Ba as a result of the reaction with tungsten in the cathode wire 3 that proceeds according to the scheme shown below, and the generated excess Ba diffuses or otherwise migrates to the surface of each cathode so as to form donors in BaO that contribute to electron emission:
ti 6BaO+W.fwdarw.Ba.sub.3 WO.sub.6 +3Ba (Reaction Equation 1).
The display device shown in FIG. 2 will operate as follows. When the cathode wire 3 is heated to about 700.degree. C. by supplying power across the wire 3, electrons will be emitted from the surface of cathodes 4b. If a positive voltage is applied to the grid electrode 6 and the anode 8, the emitted electron beams will fly through holes 6a in the grid to impinge on the phosphor 7 for its excitation. If a negative voltage is applied to the control electrode 5, the electric field around the cathodes 4b will become negative to the cathodes 4b, thereby stopping electron emission from the cathodes 4b. Therefore, the emission of electron beams from the cathodes 4b can be controlled by applying a positive pulsive voltage to the control electrode 5.
In the conventional hot cathodes in a wire form, excess Ba is generated only by the reaction between BaO in the electron-emitting material and a heat-resistant metal, i.e., tungsten. The amount of excess Ba generated by this reaction is too small to avoid the suppression of electron emission by impurity gases. During the heating of the ternary carbonate on the cathode wire for its conversion to an oxide form or during the initial operation of the display device, impurity gases will be liberated from the phosphor 7 and the supports 2 so as to decrease the initial emission current. The supply of Ba also becomes insufficient after prolonged operation and this again leads to a reduced emission current. Further problems with the conventional display device are that it produces a low contrast on account of reduced emission current and that it takes an undesirably long time to completely evacuate the system.